Aircraft secondary power in flight is generally provided by a main engine (prime mover engine), for example by extracting bleed air from the main engine compressors and by extracting shaft power from the main engine shaft for driving generators and hydraulic pumps. The bleed air is typically used for cabin pressurization and or de-icing. The shaft power is typically used for electrical generation and hydraulics. Secondary power extraction from the main engine, particularly when obtained in conditions of reduced thermal efficiency for example during at part load, can represent a significant portion of fuel burn.
Traditional gas turbine engine auxiliary power units (i.e. including an engine core with a combustor) typically have a thermal efficiency which is much lower than that of the main engine at high power, and are optimized to generate secondary power while on ground. Accordingly secondary power generated with such gas turbine engine auxiliary power units during flight would increase fuel burn, thus requiring the secondary power during flight to be generated by the main engine.